When cables, e.g. telephone cables are installed in ducts, there is a need to feed a rope or wire through the duct, in advance of the cable. The rope, after being threaded through the duct is attached to the cable, which is then pulled through the duct by pulling on the rope.
The duct motor of certain prior art devices suffers from the disadvantage that under high pneumatic pressures, the motor tends to "lock up" i.e. jam and refuse to work. The reason for this, is not fully understood. In addition this duct motor is long, uses a large number of components, and does not make the most efficient use of the pneumatic pressure available to drive it. The prior art duct motor, in essence consists of two inflatable bags disposed at each end of a telescopic member. The motor progresses along a duct by inflating one bag so that it grips the duct wall expanding the telescopic member, inflating the second bag so that it grips the duct wall contracting the telescopic member and so on. The cycle of events is controlled by means of a number of valves, in particular a main distribution valve, and inflation and deflation valves associated with the two gripper bags. High pressure air is delivered to one end of the duct motor, and thence passes to the appropriate gripper bag.
In the prior art duct motor, the central telescopic unit interconnecting the two bag grips (gripper bags) is a complex construction because of the need to provide an annular air space on the outside of the cylinder together with protective shields, to feed compressed air to the gripper bag remote from the compressed air supply. This means that the piston area inside the telescopic section does not approach the optimum value for most efficient use of the available pneumatic pressure, in relation to the overall diameter of the telescopic section. In addition in the prior art duct motor, the compressed air distribution valve is located between the two gripper bags, this means that the ratio of the unextended telescopic section to the extended telescopic section is substantially less than 2 to 1 i.e. the rate of advance is limited by the dead space between the gripper bags occupied by the distribution valve.